chloritoid

{{infobox mineral

| name = Chloritoid

| category = Nesosilicates

| image = Chloritoid-bem-12b.jpg

| imagesize =

| alt =

| caption = Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm)

| formula = {{chem|(Fe|,Mg|,Mn)|2|Al|4|Si|2|O|10|(O|H)|4}}

| IMAsymbol=Cld{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 9.AF.85

| dana = 52.03.03.01

| system = 1A polytype: triclinic
2M polytype: monoclinic

| class = 1A polytype: pinacoidal ({{overline|1}})
2M polytype: prismatic (2/m)

| unit cell = 1A polytype: a = 9.46 Å,
b = 5.50 Å, c = 9.15 Å;
α = 97.05°, β = 101.56°,
γ = 90.10°
2M polytype: a = 9.50 Å,
b = 5.50 Å, c = 18.22 Å;
β = 101.9°; Z = 4

| color = Dark gray, greenish gray, greenish black

| colour =

| habit = Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliae

typically curved or bent; also massive

| twinning = Common on {001}, polysynthetic may be lamellar

| cleavage = Perfect on {001}, distinct on {110}; parting on {010}

| fracture =

| tenacity = Brittle

| mohs = 6.5

| luster = pearly on cleavage surfaces

| streak = White, grayish, or very slightly greenish

| diaphaneity = Translucent

| gravity = 3.46 – 3.80

| density =

| polish =

| opticalprop = Biaxial (+) or (−)

| refractive = n_α = 1.713 – 1.730 n_β = 1.719 – 1.734 n_γ = 1.723 – 1.740

| birefringence = δ = 0.010

| pleochroism = X = olive-green to yellow; Y = grayish blue to blue; Z = colorless to pale greenish yellow

| 2V = Measured: 36° to 89°

| dispersion = r > v; strong

| extinction =

| length fast/slow =

| fluorescence =

| absorption =

| melt =

| fusibility =

| diagnostic =

| solubility =

| impurities =

| alteration =

| other =

| prop1 =

| prop1text =

| references =

}}

Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula {{chem|(Fe|, Mg|, Mn)|2|Al|4|Si|2|O|10|(O|H)|4}}. It occurs as greenish grey to black platy micaceous crystals and foliated masses. Its Mohs hardness is 6.5, unusually high for a platy mineral, and it has a specific gravity of 3.52 to 3.57. It typically occurs in phyllites, schists and marbles.

Both monoclinic and triclinic polytypes exist and both are pseudohexagonal.[http://rruff.geo.arizona.edu/doclib/hom/chloritoid.pdf Handbook of Mineralogy][http://www.mindat.org/min-1017.html Mindat]

It was first described in 1837 from localities in the Ural Mountains region of Russia. It was named for its similarity to the chlorite group of minerals.[http://webmineral.com/data/Chloritoid.shtml Webmineral data]